1. Field of the Invention
This invention relates to a control system for an internal combustion engine.
2. Description of the Related Art
Conventionally, there has been proposed a control system of this kind e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2005-344550. This control system is applied to a diesel engine for an automotive vehicle, and when the engine is idling, the control system estimates a cetane number of fuel during execution of a pilot injection and a main injection. The estimation of the cetane number is carried out, based on a heat release amount parameter calculated according to pressure in a combustion chamber detected by an in-cylinder pressure sensor and the volume of the combustion chamber calculated from a detection value of the crank angle sensor, in the following manner:
First, by controlling the amount of fuel injected by pilot injection, the difference between the maximum value and the minimum value of the heat release amount parameter assumed during a time period until main injection is adjusted to a predetermined amount. Next, the main injection is carried out under predetermined conditions, and a rate of rise in the heat release amount parameter calculated during a predetermined crank angle section from a time point at which the heat release amount parameter started to rise by the main injection is calculated a plurality of times. An estimated value of the cetane number is calculated according to the average of a plurality of calculated values of the rate of rise in the heat release amount.
In the above-mentioned control system, the cetane number is calculated based on the heat release amount parameter calculated according to the pressure in the combustion chamber. However, in engine operating conditions other than idling, the pressure in the combustion chamber varies depending on load on the engine etc., and the heat release amount parameter also varies accordingly. As a consequence, the estimated value of the cetane number deviates from a proper value, which makes is impossible to estimate the cetane number with accuracy.
To overcome the problem, assuming that the estimation of the cetane number is carried out only during idling of the engine, if the vehicle is started almost without executing idling immediately after refueling, the cetane number cannot be estimated until idling of the engine is executed next time. Therefore, if the cetane number of replenished fuel is different from fuel used before refueling, the control of the engine is carried out based on an estimated value of the cetane number different from the actual cetane number. As a consequence, actual ignition timing of fuel varies to increase noise and vibrations, or increase exhaust emissions.